Dense bread is the most common sourdough frustration. I baked many bricks before I understood what was going wrong. In almost every case, the cause is one of four things.
1. Starter not active enough. If your starter doesn't double in 4–6 hours, it won't raise bread. Feed it twice a day for 2–3 days before baking. 2. Under-fermented dough. The bulk fermentation needs to increase volume by 50–75%. If you're proofing in the fridge, the dough should already have risen noticeably before it goes in. Cold doesn't ferment much — it mainly develops flavour. 3. Shaping too tight or too loose. The dough needs surface tension but not so much that you degassed it. Gentle but deliberate. 4. Under-baked. Sourdough needs more time than you think. When you think it's done, give it five more minutes. The crust should be deep brown, not golden.
One more thing: flour matters. Cheap all-purpose flour with 9% protein won't give you an open crumb. Look for bread flour with 12–13% protein. The difference is immediate and significant.